Large diameter pipe such as that used to transmit substances such as oil and gas is manufactured in sections that are a few tens of feet in length. Pipe diameters can vary considerably but typically are between about 5 and 72 inches in diameter, although diameters outside of that range are known. Pipelines are constructed from individual metallic pipes (sections) which are laid individually end to end and then joined to one another by means of a welded connection. Pipelines can extend for many miles in length and are expected to last for years. Additional information related to the general environment of the instant invention can be found in, for example, U.S. Pat. No. 7,077,020, the disclosure of which is incorporated herein but referenced as if fully set out at this point.
The pipe of greatest interest herein is made of steel, thus it is customary to apply some sort of coating to the interior surface of each section of pipe to help protect it against corrosion by the fluids that flow through it. Typically this coating is applied to the interior of the pipe at the factory before the pipe leaves for installation. Imperfections in the coating can, of course, lead to subsequent corrosion and, ultimately, failure in the field. These imperfections might be due to problems at the factory, subsequent handling, installation, etc. Thus, it is common and desirable to determine the status of the coating as a final step after its installation.
The point of contact between adjacent pipe sections is also a potential source of failure in the field. In a typical arrangement, pipe sections are placed end to end and welded together to form a continuous pipeline. The welding at the joints (e.g., a “girth weld” or “field joint”) is also subject to imperfections of different sorts that might have been created during the welding process. Further, the area near the end of each pipe section (e.g., “coating cutback”) is typically not coated at the factory since such coating would be destroyed or corrupted by the welding process. Thus, there will be a gap in the coating of two pipe sections at their junction and it is desirable to coat at least that portion of the inside of each pipe after welding and before beginning to move fluids (to include gasses) through the pipeline. That operation must obviously be performed from inside the pipe and robotic solutions to perform this task are well known.
In the field multiple pipe sections are welded together to form a continuous pipeline that may extend for many miles. In some cases, the pipeline might be buried or submerged (e.g., placed on the ocean floor) where it may be difficult to access subsequently. Thus, it is imperative that the coating that is applied be unbroken or otherwise the useful life of the pipe section could be radically shortened. Of course, failure of a pipe section could result in release of its contents into the environment and/or could necessitate a costly repair or replacement of that section.
Imperfections in the coating of a steel pipe are typically sensed by way of a high voltage conductivity measurement. In a conventional arrangement, a robot is sent through the pipe section trailing behind it a wire that is placed in electronic communication with an uncoated section of the pipe. The robot then applies an electric voltage to a conductor (e.g., a brush with copper or brass strands) that is in contact with the inner surface of the pipe. Since the coating is generally nonconductive, pinholes, discontinuities, and other imperfections (i.e., “holidays”) will allow a circuit to be completed which results in a lowered resistivity, thus making such imperfections sensible via conductivity measurements. Additionally, such an imperfection will typically also manifest itself as a spark between conductive brush and the pipeline wall, thereby providing a further indication of a holiday. Holidays may be marked after they are detected (e.g., by applying a small amount of highly visible paint or dye proximate to the pipe in the vicinity of the holiday) after which insertion of a second robot unit may be necessary in order to apply an additional coating to correct the problem area(s).
As is indicated above, it is conventional for such robots to drag behind them a long grounding wire which is attached to (or in electronic communication therewith) the bare steel of the pipe. This connection might be made by attaching the end of the wire opposite the robot to the bare steel of the pipe which is usually found on its exterior or on the inside of the pipe proximate to the point where the robot enters the pipeline (e.g., the outermost coating cutback end). Of course, this wire is subject to tangling or breaking and, if such happens, prior art robots must be withdrawn from the pipeline and the grounding wire repaired. Such removal and repair can take a considerable amount of time and, as might be suspected, a delay in completion of this stage of the pipeline construction will result in money lost to the operator.
Finally, a conventional approach to searching for holidays proximate a girth weld involves the use of a robot that has a conducting brush affixed to a rotating arm. As might be expected, in practice a charge is applied to the brush as it is swept through a 360° (more or less) arc. However, such an arrangement is not suitable for testing the entirety of the interior of the pipe. Further, a rotating arm is subject to a number of potential mechanical problems and, if such occurs, the robot will need to be withdrawn from the pipe and repaired. Such delays, of course, only increase the cost of the pipeline for the operator.
Thus, what is needed is an apparatus for locating holidays in coated pipe proximate a girth weld that does not suffer from the disadvantages of the prior art. It would be preferred that such a system would not utilize a grounding wire. Additionally, a new method of detecting holidays throughout the length of the pipe is needed that does not employ a rotating arm.
Heretofore, as is well known in the pipeline coating inspection arts, there has been a need for an invention that was not subject to the problems evident in the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a system that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of the invention within the ambit of the appended claims.